Prior art off-road bicycles include one in which the front wheel is supported by a front fork provided with a buffering mechanism and an attenuating mechanism. FIG. 4 is a front fork, similar to that discussed in U.S. Pat. No. 4,971,344 comprising two front fork assemblies 1, each having an inner and an outer tube 2 and 3 which are slidable relative to each other. The inner tubes 2 of the front fork assemblies 1 have their upper end coupled to a steering bracket 4. The outer tubes 3 are coupled by mounting bolts 9 to a cross member 5, each having their lower end formed with an axle support 6 for supporting the axle.
The axle supports 6 each have a support surface, which is perpendicular to the axis 7 of the axle (i.e., perpendicular to the plane of paper of FIG. 4). Each outer tube 3 has a cross member mounting portion 8, which has a mounting surface extending parallel to the axis 7 of the axle (i.e., parallel to the plane of paper of FIG. 4). This means that the directions of a cutting tool for machining the support surface of the axle support 6 and that for machining the mounting surface of the cross member mounting portion 8 differ by 90 degrees. Therefore, the direction of the cutting tool has to be changed for machining the support and mounting surfaces.
During steering, the front fork assemblies 1 receive twisting forces F exerted from the front wheel axle. These twisting forces F are supported by the cross member 5. The twisting forces F act in a direction parallel to the axis 7 of the axle (i.e., parallel to the plane of paper of FIG. 4), and are supported by the sections of the mounting bolts 9 perpendicular to the axis thereof.
Further, where a brake lever (not shown) is coupled to the cross member 5, brake reaction forces N are applied to the cross member 5 at the time of a braking operation. These braking reaction forces N are applied in the direction of insertion of the mounting bolts 9 (i.e., direction perpendicular to the plane of paper of FIG. 4). That is, they are supported by the head of each mounting bolt 9 having a small support area. Therefore, with the prior art cross member 5 mounting structure, the mechanical strength is relatively low with respect to the twisting and brake reaction forces F and N, and the rigidity is insufficient.